Sharks are surely one of the most fearsome creatures on
Earth. But they are also one of the most sophisticated and
enduring. The oldest shark fossils date from more than 300
million years ago, before the age of the dinosaurs. And a few
existing shark species, such as the hornshark, have kept the
same basic physical characteristics for more than 150 million
Photo courtesy Carl
Roessler A rare, tiger
ragged-tooth shark, photographed off the coast of
Scientists attribute this remarkable longevity to the
shark's superior physiological developments. Sharks are
outfitted with several special characteristics that make them
highly effective, both as hunters and survivors. In this
edition of HowStuffWorks,
we'll examine these unique qualities to see why the shark is
top dog in the ocean.
Shark Anatomy When people think of sharks,
they generally imagine an animal like the one in this picture.
The tall dorsal fin, torpedo-shaped body and giant teeth of
this great white shark are familiar to most everybody.
But there are actually more than 400 different shark species
alive today, and they vary considerably in size and
appearance. In fact, roughly 50 percent of shark species are
less than a meter long. So what makes a shark a shark?
Photo courtesy Carl
Roessler One of the most
well-known sharks, the great
Photo courtesy Carl
Roessler A cat
shark, characterized by a stocky body and flowing
Sharks come in all shapes and sizes.
One of the smallest sharks, the spined pigmy, is only 6
inches long when fully grown. The largest shark, the
whale shark, can grow to more than 40 feet long. Highly
active sharks tend to be torpedo-shaped, but many less
active sharks are fairly flat, like rays.
Sharks are also very diverse in their habits and
habitats. They live all over the world and swim at every
ocean depth. Larger, more active sharks tend to hunt in
the middle and upper depths of the ocean, while many
smaller sharks stay near the ocean bottom. Some sharks
swim long distances every day, while others live a
relatively sedentary life, sticking to a small area.
with rays and chimeras, are distinguished from
other fish primarily by their body composition. Most other
fish have skeletons made of bone, just like mammals, reptiles,
amphibians and birds. Sharks and rays, on the other hand, have
skeletons made entirely of cartilage, the same flexible
material in your nose and ears. Cartilage is sturdy like bone,
but it has a much lower density. This material keeps sharks
relatively lightweight, so they don't sink in the ocean and
they don't need an air bladder like other fish.
Sharks also have a very unique skin texture. They don't
have the large, prominent scales found in bony fish. Instead
they're covered with smaller, tooth-like scales called
denticles. These tough, protective denticles are
aligned so that they channel water over the shark's body,
minimizing drag due to friction.
Like bony fish, sharks breathe by extracting dissolved
oxygen from water. The water enters the mouth, passes through
the gills and is expelled through gill slits
behind the head. In bony fish, these slits are covered, but in
most sharks you can see them clearly. As the water flows
through the gill opening, it passes tiny gill
filaments. These filaments are covered with microscopic
blood vessel capillaries, which have a lower oxygen content
than the water around them. This imbalance causes oxygen in
the water to diffuse into the shark's bloodstream, where it is
distributed throughout the body.
Photo courtesy Carl
Roessler A nurse shark
photographed off the coast of Australia: Nurse sharks,
which hunt mostly at the ocean bottom, have a gill pump
that lets them breathe without moving through the
Some sharks have a gill pump, a set of muscles that
suck in water and push it past the gills. This works something
like our lungs -- the shark can continuously gather oxygen
while it is in a still position. Most sharks also extract
oxygen using ram ventilation, passing water over the
gills by moving forward. Some highly-active sharks depend on
ram ventilation almost entirely , which means they stay in
motion most of the time!
Sharks also differ from most bony fish in the way they
move. In the next section, we'll find out how sharks swim so
quickly and gracefully through the ocean.
Most fish depend
on the law of averages for reproduction. They produce a
huge number of offspring, but only a small percentage
will survive. Sharks, on the other hand, mate only
occasionally and give birth to a small number of babies
at a time. Female sharks are extremely particular about
choosing a mate, and males may have to contend with
tough competition. This helps ensure that the babies
will be healthy.
The birthing process varies from species to species.
Some sharks lay eggs in protected areas on the ocean
floor. When the shark pup is fully developed, it breaks
through the tough egg casing and heads out on its own.
In most shark species, the pups develop inside the womb
and are fully formed at birth. These shark pups
typically look like adult sharks, except, of course,
bony fish have a special swim bladder that helps them
move around in the water. When the fish takes in oxygen, it
can release some of the gas into the bladder. This increases
the fish's buoyancy, so it rises through the water. To
sink down to the bottom, the fish squeezes some of the gas out
of the bladder, decreasing its buoyancy. In this way, a fish
is something like a blimp or hot
air balloon that uses the upward lift of atmospheric
buoyancy to change altitude.
A shark is more like an airplane.
It doesn't have a swim bladder, so it uses its forward
movement to control vertical position. The tail is like the
shark's propeller -- the shark swings it back and forth to
move forward. In an airplane, this forward movement pushes air
around the wings. In a shark, this forward movement pushes
water around the fins. In both cases, this movement of matter
creates lift -- the fluid is different, but the principle is
exactly the same.
Photo courtesy Carl
Roessler A silky shark
photographed in the Red Sea: Silky sharks' long graceful
bodies make them excellent
Sharks have two sets of paired fins on the sides of their
body, in the same general position as the main wings and
horizontal tail wings of a plane. The shark can position these
fins at different angles, changing the path of the water
moving around them. When the shark tilts a fin up, the water
flows so there is greater pressure below the fin than above
it. This creates upward lift. When the shark tilts the fin
down, there is greater pressure above the fin than below it.
This pushes the shark downward. (To learn more about lift, see
The shark also has one or two vertical dorsal fins
on its back and sometimes a vertical anal fin on its
underside. These fins work like the vertical stabilizer wing
on an airplane. They help the shark keep its balance as it
moves through the water and they can be moved from side to
side to turn the shark left and right.
Photo courtesy Carl
Roessler Two silvertip
sharks off the coast of Papua New Guinea: Silvertip
sharks can grow as large as 10 feet
This fin arrangement gives sharks amazing maneuverability.
They can cruise at high speeds, stop suddenly and make sharp
turns in every direction. This is one of the reasons they are
such effective hunters. They move more quickly and with
greater control than any of their prey -- most of the time, a
shark's prey doesn't even know what hit it.
Of course, before a shark can swoop in for the kill, it has
to locate its prey. In the next couple sections, we'll examine
the finely tuned senses that help sharks locate and track
Basic Shark Senses One of the main reasons
sharks are such effective predators is their keenly attuned
senses. Initially, scientists thought of sharks as giant
swimming noses. When researchers plugged the nasal openings in
captive sharks, the sharks had trouble locating their prey.
This seemed to demonstrate that the shark's other senses
weren't as developed as the sense of smell. Further research
demonstrated that sharks actually have several acute senses,
but that they depend on all of them working together. When you
take one away, it significantly hinders the shark's hunting
Photo courtesy Carl
Roessler The blue shark
is characterized not only by its distinctive color but
also its long snout. Blue sharks were once the most
common shark species, but they are now an endangered
species, due to
The shark's nose is definitely one of its most impressive
(and prominent) features. As the shark moves, water flows
through two forward facing nostrils, positioned along the
sides of the snout. The water enters the nasal passage and
moves past folds of skin covered with sensory cells. In some
sharks, these sensitive cells can detect even the slightest
traces of blood in the water. A great white shark, for
example, would be able to detect a single drop of blood in an
Olympic-size pool. Most sharks can detect blood and animal
odors from many miles away.
Another amazing thing about a shark's sense of smell is
that it's directional. The twin nasal cavities act something
like your two ears: Smell coming from the left of the shark
will arrive at the left cavity just before it arrives at the
right cavity. In this way, a shark can figure out where a
smell is coming from and head in that direction.
Sharks also have a very acute sense of hearing. Research
suggests they can hear low pitch sounds well below the range
hearing. Sharks may track sounds over many miles,
listening specifically for distress sounds from wounded prey.
In sharks, eyesight varies from species to species. Some
less active sharks that stay near the water's surface don't
have particularly acute eyesight, while sharks that stay at
the bottom of the ocean have very large eyes that let them see
in near darkness. Most all sharks have a fairly wide field of
view, however, since their eyes are positioned on each side of
the head. The most extreme example of this is the hammerhead,
whose eyes actually protrude out from the head.
Photo courtesy Carl
sharks are characterized by their wide head structure.
The sharks eyes and nostrils are positioned at the ends
Many shark species also rely heavily on their sense of
taste. Before these sharks eat something, they will give it a
"test bite" first. The sensitive taste buds clustered in the
mouth analyze the potential meal to see if it's palatable.
Sharks will often reject prey that is outside their ordinary
diet (such as human beings), after this first bite.
In addition to these familiar senses, sharks also possess
some senses we don't fully understand. In the next section,
we'll look at these senses to see how they work and how they
Extra Shark Senses In the last section, we
saw that sharks have a wider hearing range than human beings,
as well as a much better sense of smell. As it turns out, they
also use one sense we don't have at all.
The ampullae of Lorenzini give the shark
electrosense. The ampullae consist of small clusters of
electrically sensitive receptor cells positioned under the
skin in the shark's head. These cells are connected to pores
on the skin's surface via small jelly-filled tubes. Scientists
still don't yet understand everything about these ampullary
organs, but they do know the sensors let sharks "see" the weak
electrical fields generated by living organisms. The range of
electrosense seems to be fairly limited -- a few feet in front
of the shark's nose -- but this is enough to seek out fish and
other prey hiding on the ocean floor.
Water flows through the lateral line systems.
Vibrations in the water stimulate sensory cells in the
main tube, alerting the shark to prey and
Another unique sense organ is the shark's lateral
line. The lateral line is basically a set of tubes just
under the shark's skin. The two main tubes run on both sides
of the body, from the shark's head all the way to its tail.
Water flows into these main tubes through pores on the skin's
surface. The insides of the main tubes are lined with
hair-like protrusions, which are connected to sensory cells.
When something comes near the shark, the water running through
the lateral line moves back and forth. This stimulates the
sensory cells, alerting the shark to any potential prey or
predators in the area.
By themselves, none of a shark's sense organs would be
adequate for effective hunting. But the combination of all
these senses make the shark an incomparable predator. The
success of sharks is due largely to these physiological
advancements -- they are superbly built to find food. They are
also quite good at catching food, of course, as we'll see in
the next session.
Shark Teeth A shark's only real weapon is
its mouth. Like its fins and sense organs, the shark's mouth
is a highly effective physiological adaptation, perfectly
suited to its job. There are two elements that make the mouth
so effective: the teeth and the jaws.
people because some of them can, and sometimes will,
feed on human beings. Sharks do attack humans from time
to time, but the risk of attack is actually very slight.
Only a small proportion of shark species have been known
to attack, and even these species exhibit this behavior
These sharks typically attack defensively, when a
person has wandered into their environment, or
accidentally, because they've mistaken a person for
their standard prey. In both cases, chances are the
shark will flee after only one bite. They don't have any
real interest in humans as a food source.
Between 75 and 100 shark attacks are reported every
year, and fewer than 20 are fatal. Statistically, this
is in extremely low number. To put it in perspective,
more people are killed by pigs or falling coconuts every
year than by sharks.
Shark teeth are something like the teeth of land predators.
They have very sharp points that will cut into meat. Sharks
are exclusively carnivores, so for the most part they don't
need any grinders for chewing plant matter. Some bottom
dwelling sharks do have special grinding teeth for cracking
shells open, but more active sharks have teeth suited only for
eating flesh. There are a wide variety of shark teeth, just as
there are a wide variety of sharks. These teeth can be divided
into two general categories.
Many shark species, such as the goblin shark and
sand tiger, have very long, thin teeth. This structure
is well suited for catching small fish. The shark kills the
fish immediately, by piercing it with a single bite. Then it
swallows the fish whole.
Sharks that go after larger prey need a different strategy
and a different sort of tooth. They tear into their prey
several times, biting off large pieces of flesh. Sharks in
this group, which includes the infamous great white
shark, have wide serrated teeth. These teeth act something
like a hunting knife -- they cut easily through tough flesh
and bone. Many sharks have combinations of long pointed teeth
and wider serrated teeth, so they can hold their prey in place
while they cut into it.
Photo courtesy Carl
Roessler A 15-foot great
white shark: Great whites attack their prey with a
mouthful of razor sharp, serrated triangular
Shark teeth have the same basic consistency as our teeth,
but they don't sit in the mouth in the same way. Our teeth
rest in sockets, and aren't replaced after childhood. Shark
teeth are attached to the jaw by soft tissue, and they fall
out all the time. This is crucial to the shark's effectiveness
-- worn or broken teeth are continually replaced by new,
sharper teeth. In some sharks, such as the great white, these
teeth are arranged in several rows.
Click the button to see how a shark bites its
Sharks have a very unique jaw structure, which makes their
mouths especially effective weapons. In most animals, the
lower jaw moves freely but the upper jaw is firmly attached to
the skull. In sharks, the upper jaw rests below the skull, but
can be detached when the shark attacks its prey. This lets the
shark thrust its entire mouth forward to grab onto its prey.
Jaw mobility varies among different species, but all modern
sharks have this ability to some degree.
Daily Life of a Shark In recent years,
scientists have uncovered a lot of new information about shark
physiology, but the day-to-day life of sharks remains fairly
mysterious. Most shark species are very difficult to study
because they travel quickly over long distances, sometimes
deep in the sea. They live in a world that is largely
inaccessible to humans.
The largest species of shark is also one of the
least aggressive. The whale shark can grow to 45 feet or
more, but it feeds mainly on plankton. It doesn't have
the sharp teeth and aggressive hunting strategy of other
large sharks. It simply swims through the water with its
mouth open collecting the small creatures in its
We do know
that sharks are solitary animals, for the most part. They
typically live and hunt by themselves, joining up with other
sharks only in certain circumstances, such as mating. Some
sharks will form schools on occasion, however. Researchers
aren't really sure why this occurs because sharks don't really
need protection from predators and they don't feed in schools.
At this point, it's still unclear why sharks behave this way.
In any case, the occurrence is very rare. Most of the time,
sharks swim alone.
When they hunt, most sharks rely on the element of surprise
in some way. In some camouflaged bottom-dwelling sharks, such
as the various wobbegong species, this is a passive
exercise. The shark blends in with the ocean floor, waiting
for its prey. When a fish gets close enough, the shark opens
its mouth wide and swallows the fish whole.
In active hunters, the element of surprise works a little
bit differently. Great whites and other sharks that hunt
bigger animals proceed very cautiously when approaching their
prey. Once it has found a potential meal, the shark will
circle at some distance, sizing up the situation. When it is
ready, the shark moves in quickly, landing a good bite before
the animal knows what's happening. Often, this first attack is
sufficient to bring down the prey. Researchers have observed
great whites behaving this way when hunting sea lions -- they
will take one good bite, and then wait for the sea lion to die
from blood loss. This sort of hunting takes a lot of energy
out of a shark, so these species usually won't feed more than
a couple times a week. Sharks that feed on smaller prey
typically eat a few times every day.
Photo courtesy Carl
Roessler A large school
of hammerhead sharks, photographed off the coast of
On rare occasions, active sharks will cooperate in a hunt.
Researchers have observed this phenomenon primarily in
sevengill sharks. When these sharks hunt large fur
seals, they rely on strength in numbers -- one large fur seal
is too big for a single shark to take down itself. The sharks
form a wide ring around a single seal, and slowly move in.
When they get close enough, one shark will suddenly attack,
and the rest will follow. This sort of behavior sometimes
occurs in other shark species, but it is extremely rare.
Scientists also know that migration plays a big part in the
lives of most shark species. The main reason most sharks
migrate is that their food migrates. Different marine animals
gather in certain areas throughout the year, for breeding, to
lay eggs and other reasons. Sharks remember these annual
patterns and return to these areas every year to take
advantage of the population boom. Sharks will also remember
human activity when it involves food supplies. Many species
gather around fishing boats, for example, because they know
fishermen might discard extra bait and small catches.
Threats to Sharks Despite their superior
physiology and hunting skills, many shark species are now
threatened with extinction due to human activity in the ocean.
The main threats to sharks are over-fishing and
accidental bycatch. In many parts of the world, sharks
are in very high demand, for their meat, skin and cartilage,
which is used in several medicines. These shark products sell
at very high prices, making them an attractive catch to
Photo courtesy Carl
Roessler A whitetip reef
shark off the coast off the coast of Australia: People
fish whitetip reef sharks for their meat, even though it
has been known to cause severe food
Sharks mate only rarely and have a relatively small number
of babies at a time. Consequently, they can't replenish their
population quickly. Sharks also have fairly long lifespans --
on average, sharks live 25 to 30 years, and some sharks live
100 years or more. If left alone, a female will mate many
times in its life. With this reproductive pattern, the death
of every single shark obviously has a significant effect on
the shark population.
Over-fishing is actually a problem for both sharks and
humans. If humans kill too many sharks in a given amount of
time, the population will dwindle and they won't be able to
catch many sharks in the future. The only way to maintain
profitable shark fishing over time is to allow sharks to
continue to reproduce, which means decreasing shark fishing
significantly. Sharks are also killed accidentally, primarily
by long lines used to catch other fish. Researchers suggest we
must ban certain fishing methods, or some shark species will
die out at some point in the near future.
One major obstacle to conservation efforts is our ignorance
about sharks. We still don't fully understand their behavior,
their breeding habits or their migration patterns. For most
shark species, we don't even have an idea of their population
size. This makes it very difficult to organize effective
conservation methods since we can't accurately calculate safe
Sharks have persevered for hundreds of millions of years,
while thousands of other animals have come and gone. When you
consider this incredible history, and the unique physiological
characteristics found in sharks, it's clear that it would be a
great tragedy to lose any shark species. They are among the
most remarkable animals on earth, and there is still so much
we don't know about them.